1. Field of the Invention
This invention relates to a real image mode finder optical system suitable for a lens shutter camera and the like in which a finder optical system is constructed to be independent of a photographing optical system.
2. Description of Related Art
Real image mode finders are designed so that a field frame Is placed adjacent to an intermediate image plane of an objective system, which is observed through an ocular system. Thus, since a visual field as well as the field frame can be clearly recognized with little flare, the view of the visual field is very fine. As such, in recent years, the real image mode finders have been often used as finders for high-grade lens shutter cameras.
Real image mode finders which are mountable in single focus cameras are disclosed, for example, by Japanese Patent Preliminary Publication Nos. Hei 3-289611 and Hei 7-128591. Each of such finders is constructed with an objective system including a single positive lens, a prism having four reflecting surfaces for forming an erect image, and an ocular system including a single positive lens. These finders, however, are such that a distance between the objective system and the prism, namely the back focal distance of an objective lens, becomes an unnecessary space, which constitutes an obstacle to compactness of the finder.
The finder magnification is the ratio between the focal lengths of the objective system. The ocular system, and the focal length of the ocular system is practically governed by the optical path length of the prism. Hence, where a prism that has four reflecting surfaces is used, the focal length of the ocular system is so long that it becomes difficult to obtain a high magnification. In this way, with the finders set forth in the above-mentioned prior art publications, it is difficult that the compactness and high-magnification of the finder.
Japanese Patent Preliminary Publication No. Hei 6-300966, on the other hand, discloses a finder which includes an objective lens having a single reflecting surface, a prism having three reflecting surfaces, and an eyepiece. This finder, although advantageous for its compactness, lacks the ability to correct aberrations of the optical system itself and cannot completely correct aberrations because a surface, directed toward an eyepoint, of the objective lens is located close to the intermediate image plane.
Further, a finder disclosed in Japanese Patent Preliminary Publication No. Sho 63-226616 is constructed with an objective system, a first prism having a single reflecting surface, a second prism having three reflecting surfaces, and an ocular system. Since the objective system of this finder is composed of two lenses of negative and positive powers, it cannot be conscientiously said that the finder is compactly designed. In this finder, the entrance surface of the first prism is curved so that the first prism also has the function of a field lens.
In any of the real image node finders described above, it is difficult to obtain compactness, and it is further difficult to attain high-magnification design and favorable correction for aberrations at the same time.